(1) Field of the Invention
The present invention relates to lubricants for electromagnetic recording media which lubricants represent amides, imides or amide-imides in which amino radicals, imino or amino-imino radicals have been substituted onto fluorine-containing polyether derivatives.
These lubricants present a superior effect as they are capable of forming thin-film layers firmly adhering to materials such as metal surfaces, glass, ceramics, and carbon and of providing favorable lubricating properties.
(2) Description of the Prior Art
When the electromagnetic recording medium and the electromagnetic head are stationary, the condition prevailing therebetween is characterized by a sliding movement, whereas, in the operating status, the flying head floats above the medium held thereupon in a fluidized lubrication state so that lubrication is required between the floating surface of the electromagnetic head and the electromagnetic recording medium, for which purpose a lubricant is applied to the surface thereof.
It has been the conventional practice to employ, as the lubricant for this purpose, perfluoroalkylpolyether compounds such as, for example, the compounds set forth in the specifications of U.S. Pat. No. 3715378, the specifications of U.S. Pat. No. 377808, and the specifications of European Patent No. 791032170.
Said perfluoroalkylpolyether compounds are superior lubricants combining the favorable properties of a high heat resistance with outstanding chemical inertness and a low vapor pressure. Being perfluorinated compounds, all of whose hydrogen atoms have been replaced by fluorine atoms, they also possess a low surface tension and a favorable wettability but present a poor absorption on organic and inorganic materials.
In recent years, thin-film electromagnetic recording media with a high recording density have been developed, and in order to improve the electromagnetic transducing characteristics between the electromagnetic recording medium and the flying head it is desirable that the lubricant film applied to the surface of the recording medium should be as thin as possible, with the requirement being that the thickness of the lubricant film should be 20 .ANG.-50 .ANG. (0.002-0.005 .mu.m).
If the lubricant film has a thickness in excess of 100 .ANG. (several 0.01 .mu.m), the head may adhere to the surface of the recording medium. The small rotational torque caused by this sticking phenomenon will make it impossible for the medium to commence rotation. It is therefore essential that the thickness of the lubricant film applied to the surface of the recording medium should be in the range from one to several molecular layers.
In order to resist the shock or wear of a flying head by applying a lubricant layer of one to several molecular layers onto the surface of the recording medium, it is essential that the lubricant should not only have a low coefficient of friction and a low surface tension but also exhibit a good adhesion to the recording medium.
In order to improve the adhesion properties of the lubricant on to the recording medium, attempts have been made to achieve such adsorption, onto the electromagnetic recording medium, of a perfluoroalkylpolyether type lubricant by substitution of a polar group onto the end of the perfluoropolyoxyalkylene radical and by increasing the dipolarization effect of this polar radial to a level exceeding a certain value.
These attempts are documented in the specifications given in U.S. Pat. No. 4,267,238, the specifications of U.S. Pat. No. 4,268,556, and the specifications of Japanese Patent Provisional Publication No. 126627/1986.
Perfluoroalkylpolyether type lubricants having a polar radical at the end, achieve said adsorptive adhesion onto the surface of the recording medium by virtue of said polar group and retain their lubricity as a result of the perfluoropolyoxyalkylene chains extending therefrom.
Fluorine-containing lubricants have been developed as chemical substances offering a greater degree of safety and a superior suitability for use in extremely severe conditions and are extensively used in such fields as require a high chemical and heat resistance.
For this reason, the main application fields in which said fluorine-containing lubricants are being used are limited to such industrial sectors as the semiconductor industry handling low vapor pressure, nonflammable, corrosive gases, the mechanical equipment areas including bearings, mechanical conveyors, chains, and machinery in the vicinity of furnaces associated with high temperature operation.
However, the realization that fluorine-type lubricants have superior properties has led to a significant expansion of their applications to include such areas as factory automation equipment, industrial robots, computer and computer-related equipment, and electrical household appliances.
The lubricants used for these types of equipment and machinery are required to meet a multiplicity of exigencies which, to a large extent, have not been capable of being fulfilled with the fluorine-containing lubricants available until now.
Whereas perfluoroalkylpolyether lubricants have an inadequate load resistance in the low-molecular range, these perfluorinated alkylpolyether lubricants exhibit a low surface tension and a favorable wetting behavior in the high-molecular range but, due to their lacking adsorption on metal surfaces, they tend to give rise to lubricant migration with resulting lubrication deficiencies during high-speed rotation.
Thus, though these lubricants are imparted a very high degree of stability, their adsorption onto metal surfaces tends to be poor and while the lubricating effect in the lubrication of sliding movement is satisfactory in the initial period, long-term operation of the machine will result in a depletion of the lubricant amount to an inadequate level, so that the lubricating effect tends to diminish.
For this reason, it is essential to enhance lubricant adsorption and improve the wear resistance of lubricants even if this implies a certain trade-off by sacrificing their stability to a certain extent, as may be required by the particular application.